Recently, I changed the spark plugs in my 2001 Ford Explorer Sport Trac. It currently has 202k miles. I first began replacing them about a month ago, and due to some trouble during the process, I was only able to change 3 plugs. After 3 had been replaced, the truck was riding much smoother, and I saw quite an improvement in gas mileage. I changed the other 3 last weekend, and since I've lost MPG, and it isn't driving near as smoothly. The plugs I put in are Bosch Double Platinum. I checked to make sure all the plugs are firing, and they are. Does anyone know what might be the cause for this? Should I try a different brand of plugs? If so, what kind? One other thing. Since I changed the first three, I've been hearing a tickig noise coming from the driver's side of the engine. It gets louder only when I accelerate. All the plug wires are fine.